This invention relates to a process for manufacturing seamless tubes and pipes (hereinafter collectively referred to as "seamless tubes") from an inexpensive titanium alloy having improved resistance to crevice corrosion and to acids. More particularly, it relates to a process for manufacturing seamless titanium alloy tubes having improved corrosion resistance in environments inducing severe crevice corrosion or in non-oxidizing acids, which pure titanium metal cannot withstand.
Titanium has good corrosion resistance in sea water and in oxidizing acids such as nitric acid and it is widely used as a material for condensers in nuclear power stations and heat-exchanger tubes in chemical plants. However, its resistance to crevice corrosion is poor in high-temperature corrosive environments containing chloride ions. Therefore, titanium alloys containing 0.12%-0.25% by weight of palladium (Ti-0.12/0.25Pd) as specified in ASTM grade 7 or 11 (or JIS Classes 11 to 13) are recommended for use in such environments. The use of these alloys which contain expensive Pd metal in a relatively large amount is limited due to their high costs.
An attempt has been made to develop a more economical titanium alloy having resistance to crevice corrosion. Japanese Unexamined Patent Application Kokai Nos. 62-107041(1987), 62-149836(1987), 64-21040(1989), and 64-21041(1989) disclose corrosion-resistant titanium alloys which contain a relatively small amount of one or more of the platinum group metals, one or two of Ni and Co, and optionally one or more of Mo, W, and V.
In order to apply these titanium alloys to actual products, a commercial manufacturing process of the products should be established so as to make it possible to manufacture products having optimum properties efficiently. This is important since the properties of titanium and titanium alloys significantly vary depending on the manufacturing process and conditions.
Particularly in the manufacture of seamless tubes, such as for use in heat exchangers, it is impossible to provide a product having good mechanical properties and corrosion resistance unless all the steps from billet making to final heat treatment are performed under properly controlled conditions. However, since fabrication of titanium alloys into sheets and welded tubes is primarily performed under cold conditions, the optimal conditions for the manufacture of seamless titanium alloy tubes have not been investigated sufficiently in the past. Thus, there is a need to establish a process and conditions for commercially manufacturing corrosion-resistant seamless titanium alloy tubes of good quality.